How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Receive Buffer Overflow Drops in Linux Using eBPF

Enterprise routing backbones and high-speed data centres frequently rely on RFC 5925 TCP Authentication Option (TCP-AO) to protect Border Gateway Protocol (BGP) sessions and internal transport paths against spoofing, connection hijacking, and state exhaustion attacks. When TCP-AO operates over asymmetric multi-gigabit network interfaces paired with multi-queue Network Interface Cards (NICs), hardware receive ring buffers can become congested under bursty cryptographic validation workloads. If an asymmetric SYN-ACK segment encounters an exhausted receive ring buffer, the network card silently drops the incoming frame before kernel memory allocation, triggering socket retransmissions reflected in tcpi_retrans_synack. This tutorial explains how to trace these buffer overflow drops using extended Berkeley Packet Filters (eBPF).

Mechanisms of Hardware Receive Buffer Overflows in TCP-AO

Modern Linux network drivers manage packet ingestion using circular DMA descriptor rings. When high volumes of SYN-ACK segments carrying variable-length TCP-AO Message Authentication Codes arrive simultaneously, the NIC’s internal hardware parser and cryptographic offload engine must validate each header before allocating an sk_buff in host memory.

When asymmetric routing introduces latency imbalances or mismatched Key Derivation Functions (such as HMAC-SHA-1-96 versus AES-128-CMAC-96), the hardware validation queue experiences backpressure. If the hardware descriptor ring fills completely, incoming SYN-ACK frames are discarded. The Linux TCP stack detects missing peer responses and increments the tcpi_retrans_synack counter within struct tcp_info. Tracing this metric in real time allows systems engineers to pinpoint hardware queue exhaustion.

Implementing the eBPF Kernel Probe

Using the BCC framework, systems administrators can attach kernel probes to the internal TCP retransmit timer and kernel packet drop tracepoints. The following eBPF program hooks into tcp_retransmit_timer to detect SYN-ACK retransmissions caused by hardware buffer drops:

#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <net/tcp.h>
#include <linux/tcp.h>

struct overflow_drop_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 queue_index;
};

BPF_PERF_OUTPUT(drop_events);

int trace_synack_retransmit(struct pt_regs *ctx, struct sock *sk) {
    if (sk == NULL)
        return 0;

    struct tcp_sock *tp = tcp_sk(sk);
    u32 synack_count = tp->tcpi_retrans_synack;

    if (synack_count > 0) {
        struct overflow_drop_t event = {};
        event.saddr = sk->__sk_common.skc_rcv_saddr;
        event.daddr = sk->__sk_common.skc_daddr;
        event.sport = sk->__sk_common.skc_num;
        event.dport = ntohs(sk->__sk_common.skc_dport);
        event.retrans_synack = synack_count;
        event.queue_index = sk->sk_rx_queue_mapping;

        drop_events.perf_submit(ctx, &event, sizeof(event));
    }
    return 0;
}

Running the Real-Time Telemetry Consumer

Save the companion Python script to capture and display drop telemetry from user space:

#!/usr/bin/env python3
from bcc import BPF
import socket
import struct

b = BPF(src_file="tcp_ao_overflow_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_retransmit")

def print_event(cpu, data, size):
    event = b["drop_events"].event(data)
    saddr = socket.inet_ntoa(struct.pack("<I", event.saddr))
    daddr = socket.inet_ntoa(struct.pack("<I", event.daddr))
    print(f"[TCP-AO BUFFER OVERFLOW] SYN-ACK Retrans: {event.retrans_synack} | "
          f"SRC: {saddr}:{event.sport} -> DST: {daddr}:{event.dport} | "
          f"RX Queue Index: {event.queue_index}")

b["drop_events"].open_perf_buffer(print_event)
print("Monitoring TCP-AO receive buffer overflow drops... Press Ctrl+C to stop.")
while True:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        break

Mitigating Hardware Receive Buffer Drops

When eBPF traces reveal high retransmission rates on designated RX queues, correlate the findings with NIC hardware counters using ethtool -S <interface> | grep -E "rx_missed|rx_over_errors|rx_fifo_errors".

To eliminate overflow conditions, expand the hardware ring descriptor capacity using ethtool -G <interface> rx 4096 and tune driver interrupt coalescing parameters via ethtool -C <interface> adaptive-rx on to ensure prompt queue servicing under heavy traffic spikes.